1. Field of the Invention
This invention relates to a technology for discharging products from a movable-hearth furnace. In particular, this invention relates to a technology for discharging a reduced product to the outside from a movable-hearth furnace and a discharging device of the reduced product, the reduced product being obtainable such that raw materials including metallic ingredients such as ores containing minerals and dust and sludge produced in ironworks and industrial wastes are accumulated on a movable hearth together with a solid reductant, and the accumulated raw materials are reduced. Particularly, the invention relates to a technology for selecting a reduced product having a large grain size and discharging the reduced product, and a device for separating the reduced product from a reductant to be left in a furnace.
2. Description of the Related Art
Steel, which is a typical reduced metal, is generally produced in a converter or an electric furnace. For example, scrap and reduced iron are melted with heat by using electric energy, and are smelted, as needed, thereby forming steel in an electric furnace. Recently, however, the supply of and demand for scrap are tight, and high-quality steel is sought on an increasing basis. Therefore, there is a tendency to use reduced iron.
A so-called “movable-hearth-furnace” method which is a process for manufacturing reduced iron and the like is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 63-108188, in which an iron ore and a solid reductant are charged and deposited on a hearth movable in a horizontal direction, and the iron ore is heated and reduced by radiation heat transfer by a burner from above the hearth, thereby manufacturing the reduced iron. In a movable-hearth furnace used in this method, the raw material is charged and heated while the hearth is horizontally moved in a heating zone, generally, in a circle (revolving). In other words, a rotary hearth furnace is used.
In the rotary hearth furnace, pellets including an iron ore and a solid reductant are charged and heated on the hearth, thereby reducing the iron ore and producing reduced pellets. The hearth of the furnace moves in a heated furnace. Therefore, a heat-resistive material is applied on a surface of the hearth, or, sometimes, heat-resistive grains are deposited on the heat-resistive material applied on the surface, thereby protecting the surface of the hearth.
A rotary-hearth furnace as a movable-hearth furnace is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-172312. In FIG. 1, the rotary-hearth furnace includes an annular furnace 10 provided with a pre-heating zone 10a, a reducing zone 10b, a melting zone 10c, and a cooling zone 10d, and an annular movable hearth 11 is disposed in the furnace 10. A mixture 12 of a raw material such as an iron ore and a solid reductant such as coal, coal char and coke is charged and deposited on the movable hearth 11. A heat-resistive material is applied to the surface of the movable hearth 11 of the furnace 10. However, the movable hearth 11 may be protected with a layer of a solid reductant which is formed between the movable hearth 11 and the mixture 12.
Burners 13 are disposed in an upper part of the furnace 10. An oxidized material including metals, such as iron ore, deposited on the movable hearth 11 is heated and reduced by using heat of combustion of the burners 13, thereby forming a reduced material such as reduced iron, and the reduced material is further heated to be melted, thereby forming a reduced product including metals with slag. A charging device 14 for charging a raw material onto the movable hearth 11 and a discharging device 15 for discharging a reduced product are shown in FIG. 1. In the technology disclosed in Japanese Unexamined Patent Application Publication No. 11-172312, a bed of a solid reductant having a given thickness is formed by depositing the powdered solid reductant on a hearth, a bed of a metal-containing material having a given thickness is formed by depositing the powdered metal-containing material on the surface of the bed of the solid reductant, and a plurality of concavities are formed in the bed of the metal-containing material from the surface thereof to the surface of the bed of the solid reductant. The metal-containing material heated at the surface thereof is reduced and melted, and gathers in the concavities remaining in the bed of the solid reductant by the effect of surface tensity and gravity of the melted material while being divided into a metal and slag. A plurality of large grains of the metal with slag are obtained in the concavities by cooling the melted material.
The reduced product and the like produced on the movable hearth 11 are generally discharged by using the discharging device 15 from the movable hearth 11 to the outside of the furnace. A screw-feeder-type discharging device as a device for discharging the reduced product and the like from a hearth is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-172312. The screw-feeder-type discharging device disclosed therein includes a rod 15a provided with a spiral screw 15b fixed around the rod 15a, as shown in FIG. 2. The rod 15a of the screw-feeder-type discharging device is disposed substantially perpendicular to the direction of movement of the movable hearth 11 and rotates on the movable hearth 11, whereby the reduced product 16 including reduced pellets and a metal disposed on the hearth 11 is collected in a direction substantially perpendicular to the direction of movement of the hearth 11 and is discharged from the hearth 11.
The technology is characterized in that the reduced product 16 which is, for example, reduced iron as a metal not including a gangue portion can be substantially completely discharged in a transversal direction of the hearth 11 by driving the screw 15b of the screw-feeder-type discharging device while the screw 15b is substantially in contact with the upper surface of the hearth 11.
Grains of a metal with slag as a reduced product and the solid reductant can be substantially completely discharged by using the above screw-feeder-type discharging device shown in FIG. 2 in the solid-reductant-bed-type reducing-melting process. The discharged grains of the reduced product 16 and the solid reductant are classified by a screen. The grains of a metal with slag are classified as manufactured goods and the solid reductant which falls through the screen is reused in the furnace.
In this technology, a mixed raw material which is a mixture of powdered iron ore and a solid reductant is deposited in a layer on a solid reductant layer 1b disposed on a hearth 11, is reduced at a given temperature, and is heated to be melted and divided into metals and slag, whereby reduced iron not including a gangue portion is produced.
However, when the screw-feeder-type discharging device described above, which collects the produced reduced iron with a screw of the screw-feeder-type discharging device, discharges a pellet-shaped reduced product in particular, there is a risk that the driving device stops when the pellet-shaped reduce product is “bitten” or becomes wedged between the screw and the hearth, which fact acts as a pressing force applied to the hearth, thereby damaging the surface of the hearth.
By the discharging method described above, deposited substances on the movable hearth are completely discharged to the outside. However, there is a drawback in that the reduced product 16 (metals) cannot be separated from the solid reductant so as to be collected (discharged) as a manufactured good, although the substances including metals and slag deposited on the movable hearth can be completely discharged by the known technology.
Another drawback has been found in the known discharging method in that the temperature of the solid reductant is significantly reduced during classification, thereby generating thermal loss, in which the reduced product 16 and the solid reductant 65 are completely discharged to the outside for classification and the solid reductant is recharged into the furnace. The solid reductant is preferably returned into the furnace while it is hot, thereby avoiding thermal loss.